CHAPTER 2 SYNTHESIS AND CHARACTERIZATION METHODS

Transcription

1 26 CHAPTER 2 SYNTHESIS AND CHARACTERIZATION METHODS 2.1 METHODS OF SYNTHESIS FOR SEMICONDUCTOR NANOMATERIALS The properties of semiconductor nanomaterials such as optical and electrical properties are mostly dependent on the size and shape of the nanomaterials. Therefore controlling the size and shape of the nanomaterials are significantly important [16, 75-77]. Several methods have been used for the synthesis of nanomaterials. Synthesis of nanomaterials in solution method has attracted due to its simple, ease of implementation, cost effective and high degree of flexibility. The size and shape of the nanocrystals can be controlled through the manipulation of the experimental conditions such as reaction time, temperature and ph etc Wet Chemical Method In this method, the semiconductor nanostructures have been formed by the reduction and decomposition of precursors. Two important processes are involved in the growth of nanocrystals from solution. First is the nucleation and second is the growth of the nanocrystals. Size and distribution of the semiconductor nanoparticles can be controlled using surfactants. Compared to other methods, it does not require complex apparatus and sophisticated technique. In a typical synthesis of semiconducting nanocrystals, precipitation reaction is important to form the nanocrystals. The precipitation process involves a

2 27 nucleation step followed by crystal growth stages. Nucleation plays an important role in controlling the size and shape of the final product. Chemical colloidal nanocrystal synthesis involves the homogeneous nucleation occurs in the absence of a solid interface by combining solute molecules to produce nuclei. The formation of nuclei can be described by LaMer diagram as shown in Figure 2.1 [78]. As described in the LaMer diagram, for homogeneous precipitation, as concentration increases a saturation point is reached where nucleation occurs. Particle growth most likely transpires by a combination of the diffusion of atoms onto the nuclei and irreversible aggregation of nuclei. The requirements for monodispersity are evident from the LaMer diagram. The rate of nucleation must be high enough so that the concentration does not continue to climb. Instead, a burst of nuclei are created in a short period. The rate of growth of these nuclei must be fast enough to reduce the concentration below the nucleation concentration point. In this way, only a limited number of particles are created. The rate of growth must be slow enough; however, the growth period is long compared to nucleation period. This usually narrows the size distribution which results from finite nucleation period. So, by controlling these factors monodisperse semiconducting nanoparticles with different sizes can be synthesized.

3 28 Figure 2.1 LaMer s Diagram (Concentration vs. Time). (LaMer et al., 1950). The rate of growth of nanocrystals prepared by this method can also be derived from LaMer plot. According to LaMer plot for the crystal nucleation process, in which the concentration of atoms steadily increases with time as the precursor is decomposed by heating, colloidal nanocrystal formation comprises the following three steps: (i) (ii) The atoms start to aggregate into nuclei via self-nucleation as increasing the monomer concentration in the solution to supersaturation levels. Then, monomers continuously aggregate on the pre-existing nuclei or seed which leads to gradual decrease in the monomer concentration. As long as the concentration of reactants is kept below the critical level, further nucleation is discouraged.

4 29 (iii) With a continuous supply of atoms via ongoing precursor decomposition, the nuclei will grow into nanocrystals of increasingly larger size until an equilibrium state is reached between the atoms on the surface of the nanocrystal and the atoms in the solution [79]. After the formation of nuclei, the subsequent growth stages also strongly govern the final morphology of the nanocrystals. Generally, the nanocrystal growth can occur under two different regimes, either in a thermodynamically controlled or kinetically controlled growth regime. The manipulation between thermodynamic and kinetic growth regimes is thus a critical factor in determining nanoparticle shape [80]. The final nanoparticle morphology can be controlled by dictating the shape of nuclei and directing the growth of the nuclei or nanocrystals. Nuclei can take on a variety of shapes determined by the chemical potentials of the different crystallographic faces, which are in turn highly dependent on the reaction environment such as temperature and solute concentration. The nuclei shape can have a strong effect on the final nanocrystal shape, for example, through selected growth of high-energy crystal faces of the nuclei [81-83]. In the presence of a surfactant in bulk solution, the products are capped by surfactant molecules, resulting in the restriction of the particle growth as well as the good dispersibility of the product in reaction solvent. In the present work, shape controlled SnS nanostructures have been synthesized by surfactant free solvothermal technique. In this method, only thermodynamic parameters involving in the shape controlled nucleation of SnS nanocrystals. Solution based preparation method of some semiconducting nanostructures are discussed as follows. Huang et al., have reported the synthesis of flower like ZnO nanostructures through chemical solution route without using any surfactants. Flowerlike ZnO nanostructures were achieved via a two- step nucleation and growth

5 30 mechanism. Initially, ZnO nanosheets were formed through conventional nucleation and a subsequent crystal growth process. Then, ZnO nanosheets with some nanoparticles were formed via another conventional nucleation on the ZnO nanosheet and a subsequent crystal growth process. In the crystal growth process, each nanocluster in the aggregates or on the nanosheets has its own orientation and works as a nucleus for further growth. These grown progresses are related to both the anisotropic crystal structure of ZnO and the involved solution conditions. The surface energy was substantially reduced when the neighboring nanosheets were grown. As a result, flowerlike ZnO nanostructures were constructed by this two-step nucleation and growth process [84]. Liu et al., have synthesized SnS nanowires in aqueous solution using CTAB as surfactant. SnS nanowires formed through the aggregation of SnS nanoparticles [61]. Highly luminescent, nearly monodispersed ZnSe quantum dots have been synthesized through wet chemical method in aqueous solution of hydrazine hydrate and ethylene glycol at 70 C, free from any hazardous element and surfactant. Se source was derived from the reduction of Se by N 2 H 4. These highly reactive Se can be easily converted into Se 2, which results in a high monomer concentration. In the initial step, hydrazine hydrate complexes with metallic Zn 2+ and forms the transparent soluble complexes solution, which effectively decreases the concentration of Zn 2+ and avoids the precipitation of ZnSeO 3, thus providing a more homogenous solution environment for the reaction. Se 2 is released slowly and interacts with surplus N 2 H 4 to form the molecular precursor immediately [85] Solvothermal Method Solvothermal process is defined as a chemical reaction in closed systems in the presence of solvent at elevated temperature and pressure [86]. Two types of parameters are involved in the solvothermal method,

6 31 Chemical parameters and Thermodynamical parameters Chemical Parameters In the solvothermal process, the property of solvent such as solubility and the reactivity of the precursor get changed at elevated temperature. In this process, the initial concentration of the precursors plays an important role on the shape of nanocrystallites. Wang et al., have reported the synthesis of size and shape (dots, rods and branched) controlled CdSe and CdTe nanocrystals through solvothermal method by controlling the initial precursor concentrations [87]. Zhang et al., have reported the shape controlled synthesis of Antimony trioxide (Sb 2 O 3 ) nanostructures (nanorods, needle-like fibres, nanobelts and nanotubes) under solvothermal condition by varying the concentration of reactants and surfactant [88]. The selection of solvent plays a key-role in the morphological changes of the final product. Under solvothermal conditions, the reaction mechanisms depend on the physico-chemical properties of the solvent. SnS nanocrystals with different shapes such as nanowires, nanorods and nanoparticles have been synthesized through solvothermal method by choosing different solvents [7]. The interactions between reagents and solvent play an important role in the solvothermal reactions. Li et al., have described the synthesis of Cu 7 Te 4 using ethylenediamine as a solvent. Under the same experimental conditions, tellurium did not react with copper chloride when benzene is used as a solvent. Hence, solvent plays an important role in the formation of Cu 7 Te 4 [89].

7 Thermodynamical Parameters The thermodynamical parameters to be considered in a solvothermal process are temperature, pressure and reaction time. Solvothermal reactions are mainly developed in mild temperature conditions (T < 400 C) [86]. The reaction time and reaction temperature has an important role in the morphological variations of the material. Hu et al., have observed that the growth of elegant 3D SnS urchin-like architectures and its subsequent transformation into 1D SnS nanostructures strongly depend on the reaction time [57]. SnS nanostructures such as nanobelts, nanorightangles, nanorods and nanosheets have been synthesized by solvothermal method at different reaction temperatures [54]. Liu et al., have reported the solvothermal synthesis of uniform Mn 2 O 3 octahedral nanoparticles using N, N-dimethylformamide (DMF) and PVP. Mn 2 O 3 octahedral structure has been formed through the oriented aggregation of primary nanocrystals with increasing the reaction time [90]. Single-crystalline CuInSe 2 nanorods of 50 nm -100 nm in diameter and few micrometers in length have been synthesized through solvothermal method [91]. 2.2 CHARACTERIZATION TECHNIQUES The overview of the experimental techniques which are used in the research work, to investigate the structure, size, morphology and optical properties of SnS, ZnO nanostructures and SnS/ZnO nanocomposite are presented here X-Ray Diffraction (XRD) X-ray diffraction is a method used to determine the crystal structure, phase purity and crystallite size of a particular material. In the typical experiment,

8 33 X-rays are produced when a beam of accelerated electrons strikes a metal target. An inner electron from an atom in the metal is ejected, an outer electron drops down to fill the vacancy and radiation is emitted in the X-ray region. The monochromatic beam of X-ray is incident on a solid sample (single crystal or powder) of the material under investigation. A powder contains many very small crystals, in a variety of different orientations, whereas for a single crystal only one orientation of the solid can be considered at a time. X-ray diffraction and of the exact requirement for the appearance of intensity maxima was presented by W. L. Bragg in He realized that when X-rays impinge on a crystal, some are reflected from the atoms in the top layer whereas others penetrate this layer and are reflected off the next layer, and so on. Analysis shows that the resultant reflected rays are only in phase for certain angles of incidence of the X-ray upon the crystal. The path difference between two waves is given by, 2λ = 2d sinθ (2.1) For constructive interference, the path difference between these waves must be an integral number of wavelengths. Bragg equation was raised (equation 2.1) in which d is the separation between the planes, θ is the angle of incidence and λ is the wavelength of the X-rays: where n : an integer corresponding to the order of diffraction. 2d sin θ = n λ (2.2) In the present work, STOE X-ray diffractometer was used to record the powder X-ray diffraction patterns of synthesized nanostructures and nanocomposite. In the diffractometer, Cu target is used for the X-ray production. Diffractometer

9 34 circle diameter is 426 mm. Variable divergence slits and receiving slits are used. Diffracted beam monochromator made of plane graphite crystal selects CuK α radiation. Intensity of the diffracted beam is measured using 1 mm Tl doped NaI scintillation detector. Diffraction system is run by Windows based program package called WIN-X-POW. Instrumental resolution is estimated as using silicon standard. The diffraction patterns are recorded in the θ-2θ Bragg-Brentano geometry. Silicon < > wafer is used as a low background sample holder. X-ray diffraction analysis is used for the structural identification and broadening of the diffraction peaks caused by the finite size effect are used for estimating the average crystallite size d using Scherrer s formula [92] as 0.9 d = λ β cosθ (2.3) where, λ is the wavelength of X-rays used, β is the full width half maximum. Some of the experiments were carried out using PAN X-ray diffractometer with CuK α radiation (λ = nm) Atomic Force Microscopy (AFM) Atomic force microscopy is a type of scanning probe microscope. A sharp tip at the end of the cantilever scans the sample surface using piezoelectric scanners. During scanning, depending on the topography of the surface of the sample, the force (van-der-waals, electrostatic, magnetic, etc.,) acting on the tip changes and results in the position dependent deflection of the elastic cantilever. The deflection is monitored by reflecting a laser beam on the cantilever and recording the movement of the reflected beam with a position sensitive photodetector [93]. The surface topography of the synthesized sample was identified using Agilent

10 35 Technologies PicoLE Atomic Force Microscope (AFM). The powder samples were coated on a silicon substrate by spin coating method and AFM measurements were performed in the non contact mode High-Resolution Scanning Electron Microscopy (HR-SEM) High-resolution Scanning Electron Microscopy (HR-SEM) is a technique this provides high resolution three dimensional morphological and topographical information of the solid surface. When the high intensity electron beam hits a point on the sample, numerous collisions between the electron from the beam and atoms in the sample will occur, which causes it to emit secondary electron. These secondary electrons have relatively low energy and can easily collect by the detector. The detector counts the number of electrons emitted from the sample and resulting pattern produces a three dimensional image on the screen of a detector [94]. Morphology studies were carried out using CAMSCAN LaB 6 high resolution scanning electron microscopy. In a typical measurement, the sample is prepared by adhering a small amount of powder sample onto a copper stub using double-sided carbon tape or dropping a well-dispersed suspension of the sample in a methanol solvent onto the copper tape, followed by drying in desiccator. The prepared samples are then sputtered with platinum to reduce charging effect during the measurement Transmission Electron Microscopy (TEM)/High-Resolution Transmission Electron Microscopy (HRTEM)/Selected Area Electron Diffraction (SAED) Transmission electron microscopy (TEM) is a technique that is used to characterize the morphology, size and crystal structure of materials such as

11 36 nanoparticles. Electrons have wave like characteristics, with a wavelength substantially less than visible light. Since electrons are smaller than atoms, TEM is capable of resolving atomic level detail. The microstructural characteristics and selected area electron diffraction (SAED) of the samples were identified using JEOL-2000 EX II high resolution transmission electron microscope operated at 200 kv (TEM and HRTEM). In some measurements, ZEISS EM 900 transmission electron microscopy with an accelerating voltage of 200 kv was used. Energy dispersive X-ray spectroscopy (EDX) is an analytical technique used for the elemental analysis of the samples. EDX measurements were carried out using ZEISS EM 900 transmission electron microscopy. The specimen for the TEM/HRTEM/SAED study was prepared by suspending the powder sample in methanol. The powder sample was ultra-sonicated in a methanol medium and one drop of the suspension was loaded on the carbon coated grids. These samples loaded grids were dried under a lamp for over 8 hours under ambient conditions and loaded into the specimen carousel. Phase identification has been attempted based on analysis of the electron diffraction patterns UV-VIS- NIR Analysis Optical absorption is used to measure the absorption of interband transition energies in semiconductors [95]. Size dependent properties can also be observed in UV-visible spectrum, particularly in the nano and atomic scales. These include peak broadening and shift. The band gap of the material can be measured using this technique. The relation between absorbance and concentration of the sample is given by Beer-Lambert law [96] and it is given as, A = εbc (2.4)

12 37 where, A is the absorbance of the sample, ε is the absorbtivity coefficient of the material, b is the path length through the sample, C is the Concentration. The optical absorption spectra of SnS, ZnO nanostructures and SnS/ZnO nanocomposite were recorded using Perkin Elmer Lambda 5 UV-Visible spectrometer. A small amount of sample was first dispersed in deionized water using ultrasonic bath. The above solution was then transferred into a 1 cm sampling quartz cuvette for analysis, using deionized water as reference. In the spectrometer halogen lamp is used as a near IR source and deuterium lamp as a UV-VIS source. Photomultiplier tube (PMT) was used as a detector in UV-visible region and peltier cooled PbS detector was used in the near IR. Absorbance was recorded in the range of nm and nm for SnS nanostructures and SnS/ZnO nanocomposite respectively. Some of the measurements were carried out using CARRY 5E UV-VIS-NIR spectrometer Raman Scattering Spectroscopy Raman scattering is an inelastic scattering of photons by atomic vibrations or phonons. The shift in the energy gives the information of phonons modes in the system. The basic laser Raman setup consists of a laser excitation source, focusing lens, sample chamber, collection optics, dispersing system to analyze the scattered beam and a detector. Figure 2.2 shows the block diagram of the Raman scattering setup. The Raman spectrum is excited using 532 nm line of diode - pumped solid - state laser. The beam diameter of the laser, which is typically about 4 mm is focused to a spot size of ~ 25 μm using the lens L 1. This is an achromat of focal length 70 mm and is mounted on a precision xyz- translational stage. Mirror M

13 38 is a 20 mm long 4 mm wide strip cut from thin optically flat quartz and coated with silver. This can be rotated about its vertical axis and also translated along two perpendicular directions. The mirror M is at an angle of 45 to the plane of incidence of the laser beam and is used to illuminate the sample to be investigated. Sample is placed at a working distance of about 30 mm from the mirror. The mirror is also suitably adjusted to block the spot reflected from the sample. Figure 2.2 Block diagram of the laser Raman spectroscopic setup.

14 39 Distance between L 1, M and the sample are so adjusted that the beam is focused exactly on the sample. The lens L 2 is a camera lens of focal length 50 mm and has very good collection efficiency (f/1.2). Scattered light from the sample is collected and collimated by this lens and is focused onto to the entrance slit of the monochromator using another lens L 3 of focal length 400 mm Monochromator Scattered light from the sample is dispersed using 0.85 m focal length double monochromator (model from M/s. SPEX, USA). It consists of two holographic gratings each with 1800 line/mm and the dispersions of the two gratings are in additive mode. After the first stage of dispersion, the beam is focused through an intermediate slit. Opening of this slit is optimized so as to permit maximum of the selected wavelength as well as a good rejection of the stray light. For an entrance slit opening of 300 µm, about 500 µm opening of the intermediate slit is found to be sufficient. After the second stage of dispersion, the entrance slit is imaged onto the exit slit with a magnification of unity. The exit slit opening is kept same as the entrance slit. The gratings are rotated by a stepper motor controlled cosecant drive, which ensures that the rotation of the lead screw is linear in wavenumber. The spectral band pass of the monochromator depends on the selection of the slits and the wavelength Detector and Data Acquisition System The scattered light is focused on a model ITT-FW130- photomultiplier tube (PMT) which has S-20 type response. A thermoelectric cooler cools the PMT in order to reduce the dark counts. At temperature of about -50 C, the dark count reduces to zero. The PMT is operated in photon counting mode. PMT pluses typically have 20 ns width and 2 mv height. Output pulses from the PMT are further

15 40 processed by a pre-amplifier, amplifier and discriminator stage in order to reject the noise from the dynode chain emission. After pulse shaping and amplification, these are counted using a microprocessor-based-data-acquisition-cum-control system. This unit takes care of the data collection at each step and also provides the external trigger pulse to the monochromator drive for the next step. Data is stored in the temporary memory of this unit and at the end of each scan and it is transferred to a personal computer for further analysis. MicroRaman measurements were carried out with HR 800 Jobin Yvon Raman spectrometer equipped with 1800 grooves / mm holographic grating. He-Ne laser of 633 nm was used to excite the Raman spectra. The system consists of an Olympus optical microscope mounted at the entrance of the Raman spectrograph and 10x long distance objectives is used to focus the beam on the sample. The laser spot size is approximately 3 μm. The slit width of the monochromator is 300 μm which corresponds to a resolution of 4 cm -1. The back scattered Raman spectra are recorded using super cooled (<-110 C) 1024 x 256 pixels charge-coupled device (CCD) detector, in static mode over the range of 80 to 2000 cm -1 with 20 s exposure time and 20 CCD accumulations Photoluminescence Spectroscopy Photoluminescence arises due to radiative transition in semiconductors [97]. The samples PL emission properties are characterized by four parameters: (i) Intensity (ii) Emission wavelength (iii) Bandwidth of the emission peak and (iv) Emission stability.

16 41 The PL properties of a material can change in different ambient environments or in the presence of impurities. As the dimensions are reduced to the nanoscale, PL emission properties can change, in particular the size dependent shift in the emission wavelength can be observed. The released photon corresponds to the energy difference between the states therefore it can be used to study the materials properties such as band gap, recombination mechanisms and impurity levels [98]. The excitation PL spectra of semiconductor were done with the laser line of suitable wavelength, depending on the band gap of the semiconductor used. Photoluminescence energy was measured in the back scattering geometry in absolute wavenumbers, analyzed using SPEX double monochromator and detected using a cooled PMT. Some of the room temperature photoluminescence spectra were recorded using Horiba Jobin Yvon photoluminescence system comprising of Xenon lamp as excitation source, Gemini 180 as excitation monochromator, ihr 320 as emission monochromator and a cooled (150 K) CCD detector. Symphony software was used to run the system.